Subtidal algal colonization following the removal of Echinus

Abstract

1. On the seaward face of Port Erin breakwater large algae do not occur on the lowest three metres. Here the density of Echinus esculentus is high. 2. Over a three year period all Echinus were removed from a 10 m wide strip at about monthly intervals. The total number of individuals removed approached 3000. 3. One year after initial clearance the mean density of Laminaria hyperborea sporelings on the strip was 22.7/m2 and 5.1/m2 to one side of it. By the winter these had almost disappeared outside the strip and in the succeeding years second and third year plants were found only on the strip or close to its edge. Other algae were similarly affected. 4. It is concluded that the lower limit of L. hyperborea is determined at least in part by the grazing pressure of Echinus and that the urchins exercise some control over the numbers of other algae. © 1967 Biologischen Anstalt Helgoland.